As stated in U.S. Pat. Nos. 6,671,552 and 6,901,293, a wide variety of implantable medical devices (IMDs) are commercially released or proposed for clinical implantation that are programmable in a variety of operating modes and are interrogatable using RF telemetry transmissions. Such IMDs include cardiac pacemakers, pacemaker/cardioverter/defibrillators, now referred to as implantable cardioverter-defibrillators (ICDs), cardiomyostimulators, other electrical stimulators including nerve and muscle stimulators, deep brain stimulators, and cochlear implants, drug delivery systems, cardiac and other physiologic monitors, and heart assist devices or pumps, etc. Such IMDs other than monitors and drug delivery systems comprise an implantable pulse generator (IPG) and one or more electrical medical lead coupled to a connector of the IPG bearing body signal sense and/or stimulation electrodes and/or physiologic sensors for detecting a condition of the body, a body organ or other body tissue. The IPG typically comprises a hermetically sealed housing enclosing at least one battery and electronic circuitry powered by the battery that processes input signals, provides electrical stimulation and communicates via uplink and downlink telemetry transmissions with an external medical device, typically a programmer that is capable of being used to alter an IPG operating mode or parameter. The current drawn from the battery or batteries varies in relation to the IPG operating state, e.g., during sensing or stimulating time periods.
Typical batteries used in powering IMDs other than the cardioversion/defibrillation shock delivery circuitry of ICDs comprise lithium-iodine batteries having discharge characteristics described in U.S. Pat. No. 6,167,309, for example, and lithium/carbon monofluoride batteries having discharge characteristics described in U.S. Pat. No. 6,108,579, for example. It is well known that a battery's internal impedance increases with time and usage resulting in a decrease in battery terminal voltage. The voltage drop across the battery's internal impedance, which tends to act like a voltage divider circuit, increases as the internal impedance increases. The decrease in terminal voltage eventually reaches a battery “end of life” (EOL) voltage that is insufficient to power the IMD. The discharge characteristics of batteries can be expressed by curves (or equations) of internal battery impedance as a function of expended battery capacity (in terms of charge).
As noted in the above-referenced '552 patent, typically IPGs and monitors are designed to monitor the level of battery depletion and to provide some indication when the depletion reaches a level at which the IPG or monitor should be replaced. For example, pacing IPGs typically monitor battery energy and depletion and develop an “elective replacement indicator” (ERI) when the battery depletion reaches a level such that replacement will soon be needed to avoid further depletion to the EOL voltage. Operating circuitry in the pacing IPG typically responds to issuance of an ERI by switching or deactivating operating modes to lower power consumption in order to maximize the ERI-to-EOL interval, referred to in certain instances as an elective replacement time (ERT) or recommended replacement time (RRT) during which the IPG or monitor should be replaced.
In the above-referenced '552 patent, the IPG periodically makes and stores battery voltage measurements and accumulates incident (stimulation) counts, sense and stimulation channel impedance measurements, and current drain indication data that is periodically uplink telemetry transmitted during a telemetry session to an external programmer for display and analysis. A complex process is followed in the external programmer to compute an estimated past current drain (EPCD). The EPCD is the estimated average current drain from the time of the most recent past computation to the present time of computation or a shorter time period. The programmer then computes a remaining life estimate (RLE, aka as the ERT) to EOL based on the average battery voltage and EPCD.
Alternative and simpler approaches to determining an ERT or to simply determine battery charge depletion have been proposed or implemented in IPGs over the years employing measurements of battery impedance and/or current drain and known battery characteristics.
As described in U.S. Pat. No. 6,748,273, monitoring the internal impedance of the battery and comparing it to characteristic battery impedance changes during discharge is considered to be a reliable way of determining the remaining capacity and ERT of the battery. However, certain fresh batteries exhibit a low, substantially constant, internal impedance a corresponding stable voltage for a comparatively long time, and it is difficult to perform reliable measurements of the small changes in internal impedance during this time. Further U.S. Pat. No. 5,370,668 discloses an IMD in which internal battery impedance measurements are combined with periodic assessments of the loaded terminal voltage of the battery to trigger an ERI and establish an ERT. The technique disclosed in the '668 patent is adapted particularly for rejecting transients in the battery's demand as criteria for triggering an ERI.
The '273 patent also indicates that from a theoretical point of view the ideal way of determining the remaining capacity of a battery would be measurement of the charge drawn from the battery as disclosed in U.S. Pat. Nos. 4,715,381, and 5,769,873, for example. In the above-referenced '273 patent, battery impedance is measured and an impedance-based value of the remaining capacity of the battery is determined from a detected impedance increase. An analysis of the battery impedance increase is performed to determine whether the battery impedance is a reliable indicator of the remaining battery capacity and, if not, the total charge depleted from the battery is measured, and a charge depletion-based value of the remaining capacity of the battery is determined.
In the '381 patent, an IPG battery test circuit is disclosed for quantifying the consumed charge from the number of stimulation pulses emitted and from the expended pulse charge. Other losses of current, like e.g. leakage currents, are not considered. The true remaining battery capacity could then be less than the estimated remaining capacity and consequently the remaining operation time could be overestimated.
In a further U.S. Pat. No. 5,193,538, the depletion of a pacemaker IPG battery is monitored to determine the ERT before battery voltage further depletes to the EOL voltage. The battery voltage is periodically compared to a reference or threshold voltage characterized as an ERT-value that is less than full battery voltage at beginning of life (BOL) and selected to provide an ERT of about three months to EOL. It is recognized that the rate of battery voltage depletion is dependent upon the rate at which battery charge or current is consumed in any given “stimulating mode”, which appears to reference either or both of a pacing mode and pacing parameters in a given pacing mode. Stimulating modes may include a fixed rate pacing mode or a demand pacing mode of the types referenced in the Inter-Society Commission for Heart Disease Resource Code published by the American Journal of Cardiology, 34, 487, 1974 and those subsequently implemented in pacing, cardioversion, and defibrillation. In a given pacing mode, the rate of battery depletion depends on the physician programmed pacing parameters, including pulse voltage and pulse width as well as pacing rate, as well as the utilization or percentage of time that pacing is not inhibited when the patient's underlying heart rate exceeds the programmed pacing interval.
The solution proposed in the '538 patent appears to involve varying the ERT-value in dependence on the utilized stimulating mode and in dependence on the degree of utilization of previously selected stimulating modes recorded in and available from stimulating mode selector means. A higher threshold value is selected for stimulating modes with higher energy consumption and a higher degree of utilization and a lower threshold value is selected for stimulating modes with a lower energy consumption and a lower degree of utilization. Thus, an adaptation and stabilization of the ERT between the satisfaction of the ERT-value and the point in time of the EOL-value is achieved according to the utilized stimulating mode, which deviates from an assumed standard stimulating mode.
However, this solution requires current consuming, current sources or loads to establish the ERT-values and rather precise estimations of utilization or pace pulse counts to select the same to set a current ERT-value that may provide a relatively constant ERT to EOL. Also, current sources may not be stable over extended time periods.
In U.S. Pat. Nos. 4,556,061, 5,769,873 and 6,885,894, a measurement of charge depletion is provided not by measuring the voltage level or impedance of the pacemaker IPG battery, but rather by continuously measuring the electrical current drawn from the battery and integrating that measured current over an integration time period. A precision current-sensing resistor in series with the positive side of the battery provides a sense signal having a voltage that varies according to the magnitude of current being drawn during stimulation and sensing. The sense signal is integrated using a voltage-controlled oscillator (VCO) circuit and counter, which are implemented using CMOS circuitry arranged in a switched-capacitor topology. The VCO signal is in the form of a pulse sequence, where each pulse has a duration corresponding to a discrete quantity of depleted charge. The counter counts the VCO pulses to produce the measurement of the depleted charge.
The current drawn by the IPG circuitry of the '061, '873, and '894 patents varies as a function of the instantaneous operating state, and the voltage developed across the current-sensing resistor varies as a function of the current drawn by the pacing circuitry powered by the battery. The current passing through the current-sensing resistor in the interval between pacing pulses is relatively low, resulting in a relatively low voltage drop, and is relatively high during recharge of an output capacitor following its discharge to deliver a pacing pulse, resulting in a relatively high voltage drop. In order to operate the VCO during low current drain intervals between pacing pulses, it may be necessary to select a relatively high current-sensing resistor resistance. Then, during high current drain intervals, the voltage drop across the current-sensing resistor will reduce the voltage available to power the IPG circuitry. The reliability of circuit operations may become of concern during such high current drain intervals and as battery voltage depletes over time.